Ukraine’s president Petro Poroshenko has urged his nation to stay calm after an alleged incursion by Russian troops and armour, as western powers vowed a tough response to an apparent surge in Moscow’s military assistance to separatist rebels.

Russia continued to deny helping the insurgents, despite militant leader Alexander Zakharchenko saying that serving Russian soldiers were fighting among 3,000-4,000 volunteers in eastern Ukraine “instead of going on holiday”.

Ambassadors from Nato’s 28 members will hold an emergency meeting in Brussels today, after the alliance said well over 1,000 troops and sophisticated weapons from Russia were in Ukraine, assisting a rebel counter-attack.

“I will be frank, the situation is extremely difficult. But it is manageable, manageable enough for us not to panic . . . Destabilisation of the situation and panic, this is as much of a weapon of the enemy as tanks.”

Weapons

Mr Poroshenko, who has pledged to lead Ukraine towards the EU and out of Russia’s embrace, said his forces needed “modern high-tech weapons, we need means to conduct reconnaissance”. He is likely to request such assistance from EU chiefs when he meets them in Brussels tomorrow, and his officials hope the issue will be discussed at next week’s Nato summit in Wales.

“Over the past two weeks, we have noted a significant escalation in both the level and sophistication of Russia’s military interference in Ukraine,” Brig Gen Nico Tak, the head of Nato’s crisis management centre, said yesterday. “We assess that well over 1,000 Russian troops are now operating inside Ukraine . . . They are supporting separatists [and] fighting with them,” he added.

Brig Gen Tak said a rapid push by Russian-backed rebel groups – which had seen them take the coastal town of Novoazovsk – had also cut a supply line for government forces between the coast and the main regional city of Donetsk. “We have also detected large quantities of advanced weapons, including air defence systems, artillery, tanks and armoured personnel carriers being transferred to separatist forces in eastern Ukraine,” he said.

“The presence of these weapons along with substantial numbers of Russian combat troops inside Ukraine make the situation increasingly grave.”

Defending

Having made consistent gains against the rebels in recent weeks, Ukraine’s troops are now forced to focus on defending Mariupol, a major port on the Azov Sea, some 30km west of Novoazovsk.

Reinforcement reached Mariupol yesterday, amid fears the rebels and their Russian backers were intent on securing a strip of land along the Azov coast between the Russian border and Crimea, which the Kremlin annexed in March.

The US state department said Washington would consult allies about “additional tools and sanctions that we can certainly choose to put in place” against Russia.

German chancellor Angela Merkel said the EU had made clear that “if there were a further escalation, more sanctions would have to be discussed. So the issue will be on the agenda at [tomorrow’s] summit”

French president François Hollande said Russia “cannot hope to be a 21st-century power yet not respect the rules . . . If it was proved true that Russian soldiers are present on Ukrainian soil, this would be intolerable and unacceptable.”